The tissue microenvironment in which a cancer arises plays a critical role in inhibiting tumor development, but conversely can also be misappropriated by cancerous cells into providing factors that support malignant growth. Our long-term objective is to understand the molecular crosstalk between tumor and host cells, and to determine the mechanisms by which host tissues are co-opted to facilitate cancer progression. This proposal focuses on the contribution of the Cysteine Cathepsin family of degradative enzymes to tumor-host molecular interactions. Cysteine cathepsins are up-regulated in many human and mouse cancers and their increased expression is associated with malignant progression and a poor patient prognosis. Some cathepsin family members are upregulated in tumor cells, however others are additionally or exclusively provided by host cells including endothelial and innate immune cells. Using a broad-spectrum cathepsin inhibitor, we have shown that the cathepsin family is important for all stages of tumor development in the RIP1-Tag2 pancreatic cancer mouse model. Treatment with this inhibitor decreased angiogenic switching, blocked tumor growth, and significantly impaired tumor vascularization, cell proliferation and tumor invasion. Despite the important role for cathepsins in cancer progression, the molecular mechanisms by which they facilitate tumorigenesis are still largely unknown. Our hypothesis is that individual members of the cathepsin family are crucial for distinct steps in acquiring each distinct 'hallmark capability'of cancer. We will identify which of the 6 cathepsin family members upregulated in cancers are important for tumor development. We will test the hypothesis that cathepsins promote tumor invasion by a molecular mechanism requiring the extracellular cleavage of E-cadherin, and facilitate angiogenesis through the degradation and release of vascular basement membrane proteins. We will also determine if the cellular source of cathepsin expression in the cancer microenvironment is necessary for its pro-tumorigenic functions. To achieve these aims, we will analyze 6 mouse gene knockouts of cathepsin family members with a combination of cell co-culture, biochemical and pharmacological experiments. These studies will provide vital insights into the biology of cathepsins in cancer, the role of the microenvironment in tumor progression, and enable the development and application of therapeutic cancer strategies based on studies in this pre-clinical mouse model.